A technique has been conventionally known, in which a UV-curable ink is ejected from nozzle holes of an ink jet head onto the recording surface of a recording medium, and the ink attached to the recording surface is cured by ultraviolet irradiation so as to prevent the ink from spreading and bleeding on the recording surface (see Japanese Laid-Open Publication Nos. 2002-137375, 2003-11334, 2003-11343, and 2003-127338, for example).
As light sources of the ultraviolet light, discharge lamps such as mercury lamps and metal halide lamps are typically used. Such discharge lamps are attached to the ink jet head, so that ultraviolet light emitted by the discharge lamps is directly applied to the recording surface of a recording medium, as disclosed in the above-mentioned Japanese Laid-Open Publication Nos. 2003-11334 and 2003-11343. Alternatively, as disclosed in the above-mentioned Japanese Laid-Open Publication Nos. 2002-137375 and 2003-127338, ultraviolet light emitted by discharge lamps is guided through optical fibers to the ink jet head, from which the ultraviolet light is applied to the recording surface of a recording medium.
However, in the conventional examples, in which the ink is cured by the discharge lamps, the distribution of the illumination of the ultraviolet light emitted by the discharge lamps varies significantly on the recording surface of the recording medium. The ink curing rate thus changes from place to place, which may cause the resulting print density to differ between portions where the ink curing rate is high and portions where it is low.
In the cases where the discharge lamps are attached to the ink jet head, the ink jet head increases in size, such that high speed driving of the ink jet head becomes impossible. To enable the high speed driving, a larger driving apparatus is required. This, together with the increased size of the ink jet head, results in an increase in the size of the entire recording apparatus.
Also, the structure in which the discharge lamps and the ink jet head are connected via the optical fibers is indeed difficult to realize, because the optical fibers are made of quartz which is vulnerable to bending. Even if such a structure is obtained, the distance between the discharge lamps and the ink jet head has to be increased very much, which consequently increases the size of the entire recording apparatus.
Furthermore, discharge lamps have a relatively short life, their maintainability is poor, and their power consumption is large. In addition, discharge lamps take a long time to emit ultraviolet light in a stable manner. Thus, certain warm-up time is necessary after the lamps are operated, and even when the lamps do not have to be operated, such as when a recording medium is being carried, the lamps have to remain on.